Hyperekplexia is a rare hereditary disease. It is recognised soon after birth and becomes apparent when an infant exhibits symptoms such as an exaggerated startle reflex. If stimuli such as loud noises are unexpected, an attack is triggered where the muscle tone is increased (hypertonia) and the body becomes stiff. The classic indication of hyperekplexia is the extended startle reaction as a response to tapping of the nose. Other symptoms of the disease can include seizures and hypnagogic myoclonus. This describes twitching or jerking of the limbs during sleep. The disease can lead to many complications such as a hernia caused by the pressure increase during an attack or more serious breathing problems- apnea, which can ultimately result in death.

The diagnosis of hyperekplexia can be confusing as some symptoms mirror those of epilepsy although research has shown that the electrical activity when measured with EEG does not show the common pattern seen with epilepsy. Doctors may find it difficult to differentiate due to the rarity of the disease; they will not have much experience of it.

Hyperekplexia has been linked to mutations in the GLRA1. This was demonstrated by using a GLRA1 antagonist in a mouse which simulated the startle response similar to the reaction seen in humans with hyperekplexia. The GLRA1 gene is responsible for part of the production of the glycine receptor protein. When glycine binds to the glycine receptor protein the ligand-gated chloride channel opens allowing an influx of chloride ions and hyperpolarization to inhibit the cells signalling. The mutations that have been discovered in individuals with hyperekplexia appear to interfere with the binding mechanism and chloride channel function.